There is a plasma processing apparatus that excites plasma within a processing container using a microwave oscillator configured to oscillate microwaves. As for the microwave oscillator, a magnetron, which is inexpensive and is capable of oscillating high-power microwaves, is frequently used.
However, the frequency of microwaves oscillated by the microwave oscillator (hereinafter, referred to as an “oscillation frequency”) may be changed from a desired target frequency due to various factors. For example, the microwave oscillator is a mechanically machined product. Thus, the oscillation frequency may be changed from a desired frequency due to a mechanical error existing among a plurality of microwave oscillators. In addition, since the microwave oscillator has a frequency dependency on an output power, the oscillation frequency may be changed depending on a magnitude of the output power. Sometimes, the oscillation frequency may be changed from a desired frequency due to aged deterioration of the microwave oscillator.
To this end, techniques for regulating an oscillation frequency changed from a desired frequency are variously reviewed. For example, there is a technique in which an impedance generator is provided at a rear end of a microwave oscillator to change an impedance applied to the microwave oscillator, thereby regulating an oscillation frequency to a desired frequency. There is another technique in which a reference signal is introduced into a microwave oscillator to fix the oscillation frequency of the microwave oscillator to the frequency reference signal which has a frequency equal to the desired frequency and a power lower than the output power of the microwave oscillator. See, for example, International Publication WO 2004/068917, Japanese Patent Laid-Open Publication No. 2007-228219, Japanese Patent Laid-Open Publication No. 2002-43848, Japanese Patent Laid-Open Publication No. 2006-94214, Japanese Patent Laid-Open Publication H5-144381, Japanese Patent Laid-Open Publication H5-299024, Japanese Patent No. 4528870, and International Publication WO 2013/125260.